Larval development of Lynceus brachyurus (Crustacea, Branchiopoda, Laevicaudata): redescription of unusual crustacean nauplii, with special attention to the molt between last nauplius and first juvenile

The larval development of "conchostracans" has received only scattered attention. Here I present the results of a study on the larval (naupliar) development and the metamorphosis of Lynceus brachyurus, a member of the bivalved branchiopod order the Laevicaudata. Lynceus brachyurus is the only species of the "Conchostraca" in Denmark. The phylogenetic position of the Laevicaudata has traditionally been a source of controversy, and this study does not solve the question completely. This work focuses on features potentially important for phylogeny. The general appearance of the larvae of L. brachyurus has been known for more than a century and a half, and some of its unique features include a large, larval dorsal shield; a huge, plate-like labrum; and a pair of immovable, horn-like antennules. However, many details relating to limb morphology, potentially important for phylogeny, have not been studied previously. Based on size categories, five or six larval stages can be recognized. The larvae approximately double their length and width during development (length: 230-520 microm). Most morphological features stay largely unchanged during development, but the antennal coxal masticatory spines are significant exceptions: they become bifid after one of the first molts. In all larval stages only the antennae and the mandibles actively move. In late naupliar stages the trunk limbs become visible as rows of laterally placed, undeveloped, and still immovable lobes. Swimming is performed by the antennae, whereas the mandibles appear to be involved mainly in feeding, as in other branchiopod larvae. The last naupliar stage undergoes a small metamorphosis to the first juvenile stage, the details of which in part were studied by following the premolt juvenile condition through the cuticle of the last stage nauplius. Among other changes there is a characteristic change in the shape and morphology of the univalved dorsal naupliar shield to a bivalved juvenile carapace. The general morphologies of the antennae and the mandibles are very similar to those of other branchiopod larvae and fall well within the "branchiopod naupliar feeding apparatus" recognized as a branchiopod synapomorphy by Olesen (2003), but some specific features shared with the larvae of other "conchostracans" are also identified. These special "conchostracan" features include: 1) a similar antennular setation; 2) a similar comb-like setulation of the bifid antennal coxal processes; and 3) mandibular palpsetae with setules condensed. In light of recent suggestions concerning branchiopod phylogeny (Cyclestheria as a sister group to the Cladocera), these similarities probably do not support a monophyletic "Conchostraca" but rather are symplesiomorphies of this taxon. A final decision must await a phylogenetic analysis of a more complete set of characters.